J. S. Y. Chen

Quantitative broadband chemical sensing in air-suspended solid-core fibers

We report a sensitive evanescent field sensor using air-suspended solid-core fibers. Excellent agreement between measured and calculated mode profiles allows us to measure quantitative broadband absorption spectra with sample volumes as low as 1 muL.

Photochemistry in Photonic Crystal Fiber Nanoreactors

We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm(-1) in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin B(12), cyanocobalamin (CNCbl) to hydroxocobalamin ([H(2)OCbl](+)) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore, the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly--in our experiments one thousand times faster than in a conventional cuvette.

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells.

Fluorescence spectroscopy in photonic crystal fiber

Photonic crystal fiber (PCF) greatly enhances light-matter interactions at path lengths much longer than those in conventional sample cells. The overlap between guided modes and fluid samples introduced into the hollow channels in PCF permits quantitative absorption spectroscopy with small (<1 µL) sample volumes [1–2]. Recently, enhanced fluorescence detection was demonstrated in index-guiding PCFs surface-coated with fluorescent antibodies [3] and fluorophores [4] as well as in hollow-core PCFs in which only the hollow core is filled with liquid [5]. This last example results in highly multimode guidance by total internal reflection, yielding transverse intensity patterns that are difficult-to-control and in general axially varying.

Particle guidance and photochemistry in hollow-core photonic crystal fibre

We discuss two recent applications of hollow-core photonic crystal fibre.

Photochemistry in photonic crystal fibers

Photonic crystal fiber (PCF) has proven very useful for enhancing light-matter interactions, offering interaction lengths much longer than those available using conventional techniques. A well-defined optical mode propagating through a microfluidic channel or gas cell offers a unique way of carrying out absorption spectroscopy in very small sample volumes (∼1 µL) [1–3]. Additional advantages of PCF include its flexibility and the opportunity for system miniaturization. In this paper, we demonstrate the use of hollow-core photonic crystal fiber (HC-PCF) as a highly-controlled (photo)chemical microreactor in which reaction dynamics can be monitored in real-time via broadband spectroscopy. Strong confinement of both sample and light in the core region results in enhanced reaction dynamics and strongly reduced laser power requirements.

Controlled Particle Guidance in a Liquid-Filled Single-Mode Hollow-Core Photonic Crystal Fiber

We present controlled optical trapping and guidance of silica microparticles in the fundamental mode of D2O-filled hollow-core PCF, and show that a particle can be held stationary against an opposing fluid flow using optical propulsion.